This application is a U.S. National Stage of International application PCT/FR00/02982, filed Oct. 26, 2000 and published on May 3, 2001 in the French Language.
The present invention relates to the demodulation of amplitude-modulated signals of the type received by a portable contactless object reader after amplitude modulation of the fundamental signal transmitted by the reader to the portable contactless object, and relates particularly to such a device having a low production cost.
At present, systems designed to identify portable contactless objects are widely used in numerous applications. One of these applications is the contactless ISO card which is being increasingly used in various sectors. In the transport sector, such cards have been developed to provide users with subscription possibilities and to simplify toll booth payment operations.
Portable objects having dimensions different than those of the ISO card have been considered, such as a small transport ticket, for example. These portable objects of varying dimensions require very different antennas and thus optimal distances between the reader and the object. These distances vary depending on the object. Furthermore, the presentation of various types of portable objects in front of the same reader may take place at a variable distance. In this manner, an ISO format card may be presented at a reading distance between 0 and 8 cm while a ticket type transport voucher requires that it be placed at a distance ranging from 0 to 3 cm.
While the contactless terminal or reader emits an amplitude-modulated signal having a constant factor (10%, for example), the contactless object responds by a signal having the same fundamental frequency, amplitude-modulated at a modulation factor which is not constant. The modulation factor is the ratio between the energy absorbed by the contactless object and the energy emitted by the reader. Unfortunately, while the energy emitted is constant, this is not the same for the energy absorbed, which depends on:
the distance between the contactless object and the terminal, the energy emitted by the terminal becoming weaker as the distance increases; the same holds true for the energy absorbed by the contactless object, and
the size of the antenna enabling the object to be remotely powered and to communicate. For a given inductance, the higher the size of the antenna, the higher the absorption by the contactless object. The modulation factor thus varies depending on the position of the contactless object.
At present, several systems may be used to demodulate an amplitude-modulated signal. A traditional device allowing an amplitude-modulated signal to be demodulated uses peak detection and consists of a diode in series with a parallel RC circuit. While this system is relatively inexpensive, it has a drawback in that the values of R and C depend on the modulation factor. In other words, if this device is efficient for a given modulation factor, it becomes inefficient if the modulation factor varies as in the case of the abovementioned contactless systems.
Another demodulation device uses a mixer. It multiplies the amplitude-modulated fundamental signal by the unmodulated fundamental signal. After filtering, the modulating signal is isolated from the rest of the signal. Unlike the previous device, this device enables demodulation regardless of the modulation factor. However, it has the drawback of requiring, in addition to the mixer circuit (generally a transistor), a phase lock loop (PLL) which enables unmodulated phase generation of the fundamental signal. Unfortunately, the production cost of such a device is very high due to the fact that the cost of the phase lock loop is 100 times higher than that of the mixing circuit.
Finally, document WO/42948 describes a complex device for demodulating an amplitude-modulated signal by combining a first demodulator featuring 2 field effect transistors to act on a 100% modulated signal and a second modulator to act on a signal modulated less than 100%, this second modulator having not less than 7 field effect transistors.
The object of the invention is thus to supply a simple device for demodulating an amplitude-modulated signal regardless of the modulation factor applied during modulation.
Another purpose of the invention is to supply a device for demodulating an amplitude-modulated signal using only one transistor.
Yet another purpose of the invention is to supply a device for demodulating an amplitude-modulated signal transmitted by a contactless object to a reader in a contactless portable object identification system used as an access system in a transport network, for example.
The object of the invention is thus a device for demodulating a modulated signal resulting from the amplitude-modulation of a sinusoidal signal having a frequency fo using a signal having a frequency fm according to an unknown modulation factor, such a device featuring a transistor, the grid of which is connected to the modulated signal in order to obtain at the transistor""s output a derivative signal equal to a power N (N being an integer greater than 1) of the modulated signal and a filter connected to the output of the transistor designed to select the frequency component fm in the transistor""s output signal.
According to a preferred embodiment of the invention, the demodulation device features a field effect transistor, the signal of which includes a main component equal to the square of the input signal and an antiresonant circuit enabling the frequency component fm in the transistor""s output signal to be selected.